1. Field of the Invention
This invention relates to the shaft-seal devices, and in particular, to a shaft-seal device for insuring zero leakage along the shaft of a butterfly valve.
2. Description of the Prior Art
In the operation of large turbine apparatus, emergency conditions sometimes arise which necessitate the sudden stoppage of the steam flow through the turbine system. For example, sudden loss of an associated electrical load connected to a generator apparatus requires that the steam flow within the turbine system be interdicted in a fraction of a second to prevent turbine overspeed. After the overspeed condition has been checked, steam entrapped ahead of the closed valves must be relieved through the turbine apparatus while reestablishing normal running speed. Therefore, the valves must have, in addition to a quick closing capability, the ability of reopening and controlling the steam flow against full steam pressure drop across the valve.
In the prior art, a valve able to operate under the high temperature, high pressure, and large fluid flow conditions within a large turbine apparatus so as to completely stop such flow in a small fraction of a second and yet be further capable of reopening and controlling the rate of flow of the fluid is described and claimed in U.S. Pat. No. 3,642,024, issued to LaCoste and Dawawala, and assigned to the assignees of the present invention. The valve therein described is of the butterfly type and is comprised of a cylindrical valve body having a butterfly disc rotatably disposed on a shaft member supported within the valve body.
As is well known to those skilled in the art, steam leakage along the rotating shaft of large turbine apparatus, or along the shaft of valve members, such as the butterfly valve device of the above-mentioned LaCoste and Dawawala patent, requires that suitable seal arrangements be provided to prevent excessive steam leakage along these shafts.
In a nuclear steam power plant utilizing a Boiling Water Reactor the leakage problem is even more acute. In the Boiling Water Reactor power plant, steam generated within the reactor element itself passes directly through an associated turbine apparatus. As a result of intensive, high-energy neutron bombardment within the reactor core, the steam so transmitted through the turbine apparatus is radioactive. It is obvious therefore that excessive leakage from the turbine shaft must be prevented. In addition, the valve apparatus must also be equipped with suitable seal devices to prevent the escape of radioactively-contaminated steam from the valve shaft.
In the above-mentioned LaCoste and Dawawala patent, a single bellows seal device was disposed between the valve shaft and a low pressure gland leakoff condenser. The leakoff condenser draws air coolant used to ventilate a bearing structure required by the butterfly valve therein described. The purpose of the bellows arrangement is to prevent contamination of the air coolant by the radioactive steam within the gland condenser.
Although the seal arrangement described in the above-mentioned LaCoste and Dawawala patent eliminated much of the leakage flow, recently promulgated Atomic Energy Commission Regulations, specifically TITLE 10, CODE OF FEDERAL REGULATIONS, Part 50, require that utilities restrict radioactive waste release to the environment to levels "as low as practicable". It is therefore evident that an improved seal device is required in order to meet the recently promulgated Regulations and to insure zero leakage of the radioactively-contaminated steam along the shaft of the valve device.